This invention is directed to a recovery method for still bottom residues produced by the distillation of the spent or excess glycol resulting from the manufacture of polyethylene terephthalic by means of which antimony compound(s) that are present may be separated and, if desired, the monomers may be recovered and converted into usable materials of commercial value.
In recent years, the manufacture of synthetic fibers has grown from a point where such fibers were totally unknown to the current point in time where such fibers constitute a substantial, if not major, portion of fibers used today. Such synthetic fibers have, of course, supplanted to a large degree the use of natural fibers, such as cotton and wool, in the manufacture of clothing and other fabric products. Such synthetic fibers include polyester fibers. The recovery method of this invention is particularly directed to the recovery of materials that are normally waste by-products produced in the polyester manufacturing process.
While the production of polyethylene terephthalate resins is carried out by a number of proprietary processes, there are areas in which the methods are essentially the same. Normally dimethyl terephthalate (DMT) or terephthalic acid (TPA) is condensed with ethylene glycol to form the diglycol ester. Thereafter, in the presence of a catalyst and an increased temperature and pressure the polyethylene terephthalate is produced by splitting of methanol or water. One of the most widely used catalysts employed is antimony trioxide. Inherent in most of the processes is the recovery of the excess or "spent" glycol, normally by distillation, which, by this time is contaminated with, among other things, low molecular weight polyester chains, antimony oxide, DMT and terephthalic acid. See U.S. Pat. Nos. 2,788,373, 2,793,235 and 3,311,544 for such processes and other typical spent glycol recovery processes.
As the ethylene glycol (in the mono, di and tri forms) is fractionally distilled, most of the residual impurities remain in the bottom of the still taking the form of a waxy granular mass which is then removed as a waste material for disposal.
A typical analysis of such a still bottom, including added water and a small amount of sodium hydroxide to make the material handleable is as follows:
______________________________________ Water 26 % Solids 32 % Ethylene Glycol 36 % Di Ethylene Glycol 4.25 % Tri Ethylene Glycol .75 % Antimony 7200 PPM Sodium 4300 PPM ______________________________________
While the still bottoms described have, in the past, been disposed of by burying or dumping into a waterway, the Environmental Protection Agency has taken the position that the presence of antimony constitutes an environmental and subsequent health hazard. A maximum acceptable limit of 5 PPM of antimony has been established by the Environmental Protection Agency for effluent discharged into a waterway.
Burying can no longer be considered an acceptable disposal because the hazardous metals can leach or drain into a water supply. Pretreating for burying to insure against this or, alternatively, encasing the material becomes enormously expensive and unacceptable as a solution.
Incineration without costly accumulators and stack scrubbers simply puts the hazardous metal into the atmosphere. Ocean dumping is simply another form of waterway contamination and has a deleterious effect on marine life as well as human environment.
No known process presently exists for converting the various materials constituting the still bottoms into articles of commerce by a commercially feasible process.
It has been an objective of this invention to provide a method that will remove antimony compound(s) from polyester still bottoms.
It has been one objective of this invention to provide a recovery method for glycol distillation waste products that permits the recovery of all glycols contained in the low molecular weight polyester solids, as well as the free glycol present in still bottoms, in substantially pure form, uncontaminated by antimony.
It has been another objective of this invention to provide a recovery method of glycol distillation waste products that permits, from a commercially practical standpoint, the free terephthalic acid and the terephthalic acid to be removed from the low molecular weight polyester solid fraction in the still bottom, in substantially pure form, uncontaminated by antimony.
It has been still a further objective of this invention to provide a recovery method for glycol distillation waste products that permits separation, recovery and purification of substantially all of the antimony compound(s) contained in the still bottoms.
These objectives, taken in combination, permit the breakdown and recovery of all the usable base materials found in the still bottoms residue resulting from the distillation of spent glycol, and allows those basic raw materials to be recycled into commerce for use either once again in polyester manufacturing, or in other uses. Under certain conditions it may only be desired to remove the antimony present in the still bottoms without recovering the glycols and terephthalic acid.